This invention relates to a process for the preparation of sodium allyl sulfonate (AS) and sodium methallyl sulfonate (MAS) employing aqueous sodium sulfite solutions in an emulsion.
Sodium allyl and methallyl sulfonate are, along with other unsaturated sulfonates, important comonomers for the copolymerization with other unsaturated monomers, especially with acrylonitrile.
Sodium allyl sulfonate is produced in general by the reaction of allyl chloride with sodium sulfite in an aqueous and/or aqueous-alcoholic solution in accordance with the following reaction equation: EQU CH.sub.2 =CH-CH.sub.2 Cl+Na.sub.2 SO.sub.3.sup.H.sbsp.2.sup.O/C.sbsp.2.sup.H.sbsp.5.sup.OH CH.sub.2 =CH-CH.sub.2 SO.sub.3 Na+NaCl
The reaction is preferably conducted in an aqueous-ethanolic solution in the boiling temperature range of 42.degree.-44.degree. C. under agitation and reflux.
However, such a process gives low yields due to considerable hydrolysis and solvolysis reactions. Therefore, the selectivity of the reaction is extremely unsatisfactory. Furthermore, such a process has low space-time yields. In accordance with U.S. Pat. No. 2,601,256, reaction times of 12 hours are required. Since the solubility of Na.sub.2 SO.sub.3 in water-alcohol mixtures is low, relatively dilute reaction solutions are furthermore obtained, the working-up of which requires high evaporation costs.
Although a higher selectivity is provided by the processes of East German Pat. Nos. 70 086 and 106,828, according to which allyl chloride is introduced in the gaseous phase into an aqueous Na.sub.2 SO.sub.3 solution at, for example, 50.degree. C., wherein part of the allyl chloride is reacted and another part escapes in the unreacted state from the reaction solution and is recycled after intermediate condensation and re-evaporation, these processes exhibit a higher energy consumption as a result of the continuous evaporation and condensation. Furthermore, they require relatively long reaction periods in consequence of the poor mass transfer through the interface from the gaseous to the liquid phase.
In general, the reaction solutions are worked up, in order to obtain the sodium allyl sulfonate in the pure state, by evaporation of the solution, extraction of the AS with alcohol, and subsequent crystallization from alcohol to obtain the compound in the pure form.
There is thus lacking in the prior art a process which makes possible the production of sodium allyl sulfonate by the reaction of allyl chloride in maximally concentrated Na.sub.2 SO.sub.3 solutions in a maximally short reaction time and with high selectivity and low energy consumption.
The commercial production of MAS is conducted either at temperatures of 30.degree.-70.degree. C., preferably 65.degree.-66.degree. G., in accordance with the following reaction equation: ##STR1## or by sulfonation of isobutene with organic SO.sub.3 complex compounds in preferably inert solvents, especially halogenated hydrocarbons.
The latter method is commercially inadequate, since, on the one hand, the resultant salt has an unsatisfactory degree of purity (at most 97%) and, on the other hand, the solvent as well as the relatively expensive complexing agents must be separated from the sulfonic acid after the reaction in expensive recovery processes. In this connection, the recovery rate of the complexing agents is only about 90%, and the selectivity, based on the SO.sub.3 conversion, is at best 95%. Due to the fact that the primarily formed sulfonic acid has a great tendency to decompose, this acid cannot be isolated as such and therefore can be worked up only after neutralization to form its salt.
In accordance with the first-mentioned conventional process, aqueous Na.sub.2 SO.sub.3 solutions are normally reacted with a stoichiometric excess of MAC with the addition of solubilizers (U.S. Pat. No. 2,601,256) or emulsifiers (German Published Application DAS 1,804,135). However, it is also possible, on the other hand, to employ a quantity of MAC which is less than the stoichiometric amount, as set forth in U.S. Pat. No. 3,453,320, whereby the separation of NaCl is said to be facilitated. The yields of MAS according to these methods are only about 75% to 85%, based on the component added in less than stoichiometric amount, with reaction times of 2-12 hours.
It is also possible, as disclosed in East German Pat. Nos. 70,086 and 106,828, to introduce the MAC required for the reaction in the gaseous form in metered amounts into an aqueous Na.sub.2 SO.sub.3 solution without the addition of auxiliary agents at temperatures of above the MAC boiling point. Because of the poor mass transfer from the gaseous to the liquid phase, a portion of the thus-introduced MAC is discharged from the reactor in unreacted form, so that energy-consuming intermediate condensation and revaporization stages are required to recycle the MAC. Moreover, this process requires relatively long reaction times.
All of the aforementioned processes of reacting MAC with Na.sub.2 SO.sub.3 also have the general disadvantage that saturated sulfonates, in part polymeric and/or oligomeric MAS, are produced in addition to the desired, unsaturated sulfonate. These saturated sulfonates are practically inseparable from the MAS by conventional working-up methods, e.g., fractional crystallization or extraction, due to the fact that their characteristics are similar to those of MAS.
Consequently, there is the need for a process which permits the production of MAS by reacting MAC with an aqueous Na.sub.2 SO.sub.3 solution in a maximally short reaction time with high selectivity, without the formation of saturated sulfonates.